From the viewpoint of an improvement in turbine efficiency, an increase in temperature of a working fluid at the entrance of an axial flow turbine has been achieved. For example, in an axial flow turbine into which a working fluid having a temperature of 630° C. or higher is introduced and in an axial flow turbine from which a working fluid having a temperature of 630° C. or higher is discharged, a casing, a pipe, and the like, which are to be exposed to this high-temperature working fluid, are formed of a high-strength material such as an Ni-based alloy, which can endure the temperature of the working fluid.
Examples of a conventional axial flow turbine include one in which an outer casing is formed to cover an exhaust hood, for example. Such a formation requires the entire outer casing to be formed of a high-strength material. However, it is difficult for a large-sized cast product such as the outer casing to be formed of a high-strength material. Further, even if the large-sized cast product is manufactured of a high-strength material, it is difficult to maintain its quality.
Thus, a double-casing structure in which an inner casing surrounds the periphery where a high-temperature working fluid flows and an outer casing surrounds the periphery of the inner casing is employed. Then, the inner casing is formed of a high-strength material, and the outer casing is formed of a conventional product such as high Cr steel or low Cr steel.
In the axial flow turbine having such a double-casing structure, a double-pipe structure is employed in a path where a high-temperature working fluid is introduced into the axial flow turbine and a path where the high-temperature working fluid is discharged from the axial flow turbine. This double-pipe structure includes a short pipe formed of a high-strength material, which is welded and joined to the outer casing, and a sleeve formed of a high-strength material, which couples the short pipe and the inner casing. The high-temperature working fluid flows through between the short pipe and the inner casing via the sleeve without coming into contact with the outer casing.
A joint portion between the short pipe and the outer casing increases in temperature due to thermal radiation from the sleeve and heat conduction from the short pipe. When the temperature of the joint portion becomes higher than a predetermined temperature, a required strength in the joint portion cannot be maintained. Further, in the joint portion made by dissimilar welding of the high-strength material such as an Ni-based alloy and the conventional product, a strength reduction of a weld heat affected zone and a strength reduction due to an effect of carbon diffusion are significant. Therefore, it is necessary to maintain the temperature of the joint portion to an allowable temperature or lower.
In the conventional axial flow turbine, a technique in which a cooling fluid is introduced between the short pipe and the sleeve to cool the short pipe and the joint portion made by welding the short pipe and the outer casing is under consideration. In this conventional axial flow turbine, the temperature of the working fluid flowing through the sleeve is lower than 630° C. Further, the difference in temperature between the working fluid flowing through the sleeve and the cooling fluid is small. Therefore, even when the cooling fluid directly comes into contact with the sleeve, an effect of thermal stress is not a problem.
As described previously, the working fluid to be introduced into the axial flow turbine is increased in temperature. In the meantime, the high-temperature working fluid is sometimes discharged from the axial flow turbine. Therefore, the difference in temperature between the cooling fluid and the sleeve increases. Thereby, like the technique of cooling the joint portion in the conventional axial flow turbine, direct contact of the cooling fluid with the sleeve causes a large thermal stress.